I’ve considered writing about this subject for some time. What pushed me over the edge was seeing Powerpoint presentations in which small blocks of copy whose left-and-right justified margins made them nearly indecipherable.
I know, it’s so tempting to click that button. It makes everything neat and square. It feels like you’ve actually DONE something to your copy, instead of taking the path of least resistance and letting that raggedly old right margin hang out there all untidy.
Graphic designers may use fully justified copy for aesthetic reasons. (They’re professional drivers on a closed course. Don’t try this at home.) As you’ll read here, justification can work in documents with lines of copy longer than 40 characters, but this site and others advise you not to justify copy on the Web. I’ll go a step further and encourage you not to do it anywhere.
In left-justified copy, the ragged right edge gives the reader subtle clues about what’s ahead. It reduces hyphenation. Like Raylen Givens, the lead character in the TV series, it has a certain spontaneity that is altogether likable.